Soap helps clean in two ways: it helps water “wet” the surface to be cleaned, permitting water to reach more of the surface; and it connects the dirt to the water, permitting the dirt to rinse away. A soap molecule consists of a chain of carbon, hydrogen, and oxygen atoms — arranged with a distinct head and tail. The head is attracted to water and the tail is attracted to dirt and oil. Soap cleans because these opposing parts connect dirt to water, permitting it to be rinsed away.
Soap also helps water to wet better. Water beads up on fabric and skin because its molecules are tightly bonded and resist being broken apart. The molecules cling to one another in droplets and do not soak the surface. This is where soap intervenes as a surface-active agent — something that breaks apart these droplets and helps them wet the skin or fabric. When soap molecules are combined with water, their hydro -phobic tails (the hydrocarbon chains) squeeze together in a small space, in an effort to get as far away as possible from the water and as close as possible to one another.
The heads of the soap molecules (the carboxyl portion) are attracted to the water and form a spherical wall around their fleeing tails. The soap forms a film on the surface of the water that holds the heads and tails in position. The action of these heads and tails on the water’s surface breaks the surface tension, forces the water into the fabric or skin, and allows the soap’s lather to develop.
Once the soap molecule has helped water do its job, it next removes dirt and grease. The oil-loving tail of the soap molecule is attracted to oil and grease. It first embeds its tail into the dirt. As the water-loving head of the soap molecule pulls toward the water, the dirt is dislodged as it remains attached to the tail of the molecule. The tail of the soap molecule then holds the dirt in suspension, away from the skin or fabric, until a rinse washes the dirt and the soap away.